Sometimes the toughest tests for a public uprising take place in private. The Black Lives Matter movement faced one such moment in October, inside a turreted building of pale stone planted on the site of the old Pennsylvania Avenue slave market, just blocks from the White House. Eleven young activists filed into a conference room, snapping selfies as they waited for one of the most powerful women in America. It takes clout to finagle a meeting with Hillary Clinton—and courage to do what came next.